Recently, along development of compact information equipment, electronic components to be mounted have been downsized rapidly. A ball grid alley (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to cope with a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
As the electronic components to which the BGA is applied, for example, a semiconductor package is exemplified. In the semiconductor package, semiconductor chips having electrodes are sealed with resin. Solder bumps are formed on the electrodes of the semiconductor chips. This solder bump is formed by joining a solder ball to an electrode of the semiconductor chip. The semiconductor package to which the BGA is applied is mounted on a printed circuit board by mounting it on the printed circuit board so that each solder bump contacts a conductive land of the printed circuit board and joining the solder bump melted by the heating to the land. Additionally, a three-dimensional high-density mounting structure has been studied by stacking up the semiconductor packages in a height direction in order to meet the further high-density mounting requirement.
However, when a BGA is formed on a semiconductor package having a three-dimensional high-density mounting structure, solder balls are crushed by the self-weight of the semiconductor package, and short circuits occur between the electrodes. This may hinder for the high-density mounting.
Therefore, solder bumps using metal balls each with a very small diameter, which are made of metal such as Cu with a melting point higher than that of solder, have been studied (for example, see Patent Literature 1). The solder bump including the metal balls can support the semiconductor package by the metal balls that are not melted at the melting point of the solder even when the weight of the semiconductor package is applied to the solder bump when the electronic component is mounted on the printed circuit board. Accordingly, the solder bump is not crushed by the weight of the semiconductor package itself.